Important factors in governing magnetic properties of electrical steel sheets include sizes and dispersing conditions of Aln and MnS precipitates in steels. These precipitates are obstacles to movements of magnetic domain walls and deteriorate not only the magnetic flux density under a low magnetic field but also the iron loss. In addition, they hinder gain growth during recrystallization annealing, and immature gain growth of ferrite grains negatively influence developments of recrystallization texture preferable to the magnetic properties.
It is known that rougher and coarser precipitates are preferable for the movements of the magnetic domain walls during magnetization. Based on such background, there has been disclosed in the prior art attempts to provide coarser precipitates of AlN or MnS before the recrystallization annealing in the processes of making the electrical steel sheets. For example, Japanese Patent Laid-Open Specification 38814/74 checks disolution of the coarse AlN during slab soaking by lowering the heating temperature thereof; Japanese Patent Laid-Open Specification 22,931/81 lowers the amounts of S and O accompanying growthes of fine non-metallic inclusions; Japanese Patent Laid-Open Specification 8,409/80 controls formation of sulphides by addition of Ca or REM; Japanese Patent Laid-Open Specifications 108,318/77, 41,219/79 and 123,825/83 coarsen AlN by brief soaking of the slab before the hot rolling; and Japanese Patent Laid-Open Specification 76,422/79 utilizes a self-annealing effect by coiling at super high temperature after hot rolling for coarsening AlN and accelerating growth of ferrite grain.
From a viewpoint of saving energy in the process, it is advantageous to carry out a hot direct rolling from the continuous casting of a slab when performing the hot rolling. However, if this process is used, a problem occurs that the coarse precipitations of AlN and MnS are made insufficient. To solve this problem, the slab is subjected to the brief soaking before the hot rolling.
However, although the soaking time is short, such a process which once transfers the slab into the heating and soaking furnaces, could not enjoy the merits of saving energy brought about by the hot direct rolling. Furthermore, for providing precipitation of AlN, if the soaking time is short, the precipitation will be non-uniform at the inside and outside of the slab.